A battery system in which battery modules each including a plurality of chargeable/dischargeable battery cells are connected in parallel is known. This battery system, for example, charges the battery module with power from a power supply using natural energy such as wind power generation or solar power generation or power from a commercial power supply via a power converter such as an inverter or a converter.
As a method of charging the plurality of battery modules connected in parallel, for example, there is a method of selecting one battery module to supply power from the power supply to the selected battery module and switching a supply destination of the power to another battery module when the selected battery module has been fully charged (for example, see Patent Literature 1). In addition, a method of sequentially selecting one battery module with time and supplying the power from the power supply to the selected battery module (for example, see Patent Literature 2). In addition, a method of supplying the power from the power supply to a plurality of battery modules in parallel is also known (for example, see Patent Literature 3).